"We've known for a number of years that chronic stressful events put people at greater risk for developing a cold when they're exposed to a virus," says Sheldon Cohen, a psychologist at Carnegie Mellon University, who studies the link between stress and health. "Less clear has been why that occurs, how does the stress influence the progression of the disease."

To try to find out, Cohen and his colleagues recruited 276 volunteers who submitted to an exhaustive interview to assess the level of stress in their lives "like a difficult marriage or problems at work or losing a job, or things of that sort," Cohen says.

The volunteers were exposed to cold viruses and then put into quarantine for five days. "And when you do that, about a third of the people you expose to a virus actually develop a cold."

Cohen explains that typical cold symptoms are actually not caused by the infection itself, but instead by inflammation in response to the body's own immune system.

That immune response is supposed to regulate inflammation with a hormone called cortisol, but stress reduces the effectiveness of cortisol. So as Cohen found in the study, those who were more stressed-out were more likely to suffer severe cold symptoms when exposed to the virus.

Although Cohen's research involves the common cold, he points out that scientists are beginning to appreciate the role that inflammation plays in a wide variety of ailments, from asthma to cardiovascular disease.

"So to the extent that chronic stress does lead to this insensitivity to turning-off of the inflammatory response, that it implies that this may be a way through which chronic stress could influence all of these inflammatory diseases."